change fast cgi to mod php in ispconfig 3

While i was setting up my 2 websites on my VPS debian lenny with fresh ispconfig install. By mistake i set fast cgi for my sites because i was thinking this was better.
Right now my sites are using to much RAM so i would like to revert back to MOD PHP ... So i was thinking this is an easy job so whent to the backend of isp config and selected mod php for my sites ...
As soon as i do that i get an internal server error on my website ... I am not sure what i am doing wrong.